1. Field of the Invention
The present invention relates to an apparatus and method for performing feedback control of an air-fuel ratio of an internal combustion engine, based on signals from an air-fuel ratio sensor.
2. Description of the Related Art
In Japanese Unexamined Patent Publication No. 05-141294, an air-fuel ratio control method is disclosed in which feedback control of the injection quantity is performed based on detected values relating to the air-fuel ratio provided by an air-fuel sensor, wherein a manipulated variable is prevented from updating for a predetermined period of time immediately after fuel injection is resumed from a stopped state.
Immediately after fuel injection is resumed, the delayed response of the air-fuel ratio sensor causes the air-fuel ratio detected to be leaner than the actual ratio, and consequently, by prohibiting updating of the manipulated variable for a predetermined period of time immediately after fuel injection is resumed, any excessive increase in the quantity of correction to the injection quantity can be prevented.
However, if the transient response of the air-fuel ratio sensor deteriorates, updating of the manipulated variable begins during the period of delayed response of the air-fuel ratio sensor, and an increase in the quantity of correction to the injection quantity becomes excessive.
Here, if the length of time during which updating of the manipulated variable is prohibited, is extended so that the manipulated variable is not updated during the period of delayed response even if the transient response of the air-fuel ratio sensor deteriorates, a problem arises in that when the transient response of the air-fuel ratio sensor has not deteriorated, the manipulated variable begins to update after an unjustifiable delay, and convergence on the target air-fuel ratio is delayed.